C# Paypal定期支付api调用出现错误'；基础连接已关闭：发送时发生意外错误；

这是我试图调用api的代码

HttpWebRequest request = null;

            request = (HttpWebRequest)WebRequest.Create("https://api-3t.sandbox.paypal.com/nvp");

            request.Method = "POST";

            //string formContent = "Your Query string goes here"; // Change the post variables with values here
            StringBuilder sbUrl = new StringBuilder();
            sbUrl.Append("USER=username");
            sbUrl.Append("&PWD=****");
            sbUrl.Append("&SIGNATURE=****");
            sbUrl.Append("&METHOD=CreateRecurringPaymentsProfile");
            sbUrl.Append("&PROFILESTARTDATE=2017-02-02T00:00:00Z");
            sbUrl.Append("&DESC=RacquetClubMembership");
            sbUrl.Append("&BILLINGPERIOD=Month");
            sbUrl.Append("&BILLINGFREQUENCY=1");
            sbUrl.Append("&AMT=50");
            sbUrl.Append("&SetupPrice=150");
            sbUrl.Append("&MAXFAILEDPAYMENTS=3");
            //sbUrl.Append("&ACCT=4111111111111111");
            sbUrl.Append("&ACCT=4111111111111111");
            sbUrl.Append("&CREDITCARDTYPE=VISA");
            sbUrl.Append("&EXPDATE=022022");
            byte[] byteArray = Encoding.UTF8.GetBytes(sbUrl.ToString());
            request.ContentType = "application/x-www-form-urlencoded";
            request.ContentLength = byteArray.Length;
            Stream dataStream = request.GetRequestStream();
            dataStream.Write(byteArray, 0, byteArray.Length);
            dataStream.Close();
            WebResponse response = request.GetResponse();
            dataStream = response.GetResponseStream();
            StreamReader reader = new StreamReader(dataStream);
            //string responseFromServer = Common.UrlDecode(reader.ReadToEnd());

            reader.Close();
            dataStream.Close();
            response.Close();

---

获取“基础连接已关闭：发送时发生意外错误。

错误发生在哪一行？错误发生在哪一行？